Date: Sun, 1 Mar 1998 10:59:24 -0700 Reply-To: ji3m@maxwell.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: ji3m@maxwell.com (James R. Duffey) To: "Low Power Amateur Radio Discussion" Subject: Why Zip Cord Feedlines Are a Poor Choice Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender: ji3m@192.31.66.42 X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Zip cord antennas work, I don't think anybody said they wouldn't, but there are better ways to feed antennas than with Zip cord. Lets start with some Ham History. Back in the days before Coax was commonly (and inexpensively) available one alternate to the almost universally used balanced feedline was 72 Ohm balanced feedline. This was also in the days before plastics were common. The 72 Ohm balanced feedline looked a lot like the "lamp cord" that was used in light duty (i.e. lamps) household wiring, but was rubber insulated. The household stuff was twisted cloth covered wire. Some Hams used the lamp cord as balanced 72 Ohm feeders instead of the commercial balanced feeder. This was in the depression remember, where scrouinged parts kept many a Ham on the air who could not otherwise afford it. Getting on with higher feedline losses was more important than not getting on at all. Ham lore dies hard and when vinyl covered lamp cord (Zip Cord) became commonly available after WWII, some Hams remembered the pre war lamp cord feedlines and used the Zip Cord for feedline (and antennas). However surplus Coax was widely avaiable, and the use of Zip cord never became as big after the war as it was before the war. History lesson aside, there are several problems with using Zip cord for feedline; 1) The Vinyl insulation commonly used on Zip cord is rather high loss at HF. The losses for vinyl are much higher than the losses for the polyethylene used in coax or window line. This leads to a feedline with higher loss than even lowly RG-58U. This loss in zip cord was measured by the ARRL in the late 70s and appeared in QST at that time as well as in Handbooks until a few years ago. I forget the exact numbers, but losses are excessive above 40 M. I can look up the exact numbers and post them when I get home. 2) Manufacturing tolerances are poor for zip cord so the wire to wire spacing varies considerably. This leads to a transmission line with widely varying impedance along its length. This varying impedance will lead to a complicated reflected wave along the line, and really complicates the definition of "properly terminated" transmission line. This makes calculations of exact losses difficult. They will be higher than for a transmission line with constant spacing that is properly terminated in its characteristic impedance. 3) The Vinyl insulation used on Zip cord is designed for indoors use. It will often crack, turn brittle and depolymerize when used outdoors. Then it will be subject to wide variations in its properties when wet. The significance of this point may be difficult to grasp until you have spent an entire QSO when it is raining with one hand on the key, two hands on the antenna tuner controls, one hand on the SWR meter, and one hand on the transmitter level control. You get the point then. On 80 M the loss in Zip Cord is probably mangeable for reasonable lengths of feedlines. On 40 M and higher bands the loss increases significantly and one is much better off (lower losses) with even RG58. The losses will be very great for nonresonant antennas fed with zip cord; a 80 M dipole fed at 40 M as an example. As an emergency or experimental feedline, Zip cord can probably be justified. As an everyday feedline there are better (lower loss) choices. If you are going to Radio Shack for a feedline, even their coax will have lower losses than their zip cord (even the audiophile zipcord). I would suggest that those using zip cord feedlines for moderate runs would gain a dB or more in signal strength (more if you are using it on 20 M and higher) if they replaced the feedline portion of the zip cord antenna with a good low loss feedline such as RG213 (RG8), RG8X, or even RG58U. Window line is a good alternate for antennas to be used on multiple bands. I recognize people build zip cord antennas and are happy with them. That does mean that they are the best technology available. Even a mediocre antenna is better than no antenna at all. If you are planning a new antenna installation, and you want the best performance for your efforts, avoid zip cord. The money you save isn't worth it. There are no advantages other than cost. It is not good economics to build or buy a ham station worth several hundred dollars and then try to save ten or twenty bucks on the feedline. This isn't meant to be a flame. I just hate to see only one side of an issue presented, so I posted this. You can make contacts with zip cord antennas, but with QRP we are trying to make every milliwatt count. Zip cord will rob you of your precious power. There are better ways to feed antennas. I can address the bumble bee won't fly myth as well, but that doesn't seem to be QRP related. - Duffey KK6MC/5 James R Duffey KK6MC/5 DM65 30 Casa Loma Road Cedar Crest, NM 87008 Hope this is helpful.Dave W7AQK